Process for the dehydrogenation of hydrocarbons



Patented Aug. 7, 1945 PROCESS FOR THE DEHYDROGENATION OF HYDBOCARBONSWalter A. Schulze and John C. Hillyer, Bartlesvilie, kla., assignors toPhillips Petroleum Company, a corporation of Delaware ApplicationSeptember 7, 1940, Serial No. 355,710

8 Claims.

This invention relates to the production of the valuable-dloleflnichydrocarbons from the corresponding paraflinic and/or oleilnichydrocarbons. It relates more particularly to the process ofdehydrogenating normal pentane in successive stages to producesuccessively pentenes and pentadiene.

In a, more specific sense this invention is concerned with a new andimproved process in which the mono-olefin pentene-l, Obtained as one ofthe products from the catalytic dehydrogenation of normal pentane or ofthe C fraction of thermally cracked hydrocarbon gases is separated byfractional distillation and is subsequently dehydrogenated in a secondcatalytic step to produce a good yield of pentadiene.

This is a continuation-impart of our co-pendlng application Serial No.352,786 filed August 15, 1940, in which the preparation of butadiene isspecifically disclosed.

It has already been proposed to produce diolefins by the catalytictreatment of the total oletln mixture resulting from the dehydrogenationof normal pentane. However, it has been the practice of those attemptingto carry out such a process to first dehydrogenate pentane to producemixed pentenes and to recycle the treated vapors until the desiredconcentration of pentenes was reached or the desired percentage of thepentane charge had been converted. Because of the limitations imposed bythermodynamic equilibrium, it has not been possible to securesatisfactorily pure mixed pentenes by this proccdure without resortingto such severe conditions and excessive recycling that considerablequantities of the pentane and pentenes were decomposed, and theresulting yields of pentenes were commercially not feasible.

Also attempts have been made to produce mixed pentenes bydehydrogenating normal pentane and separating the resulting pentenesafter each passage over the catalyst for further dehydrogenation. Suchattempts have involved various complex solvent extraction and chemicalseparation methods which have proved expensive and generallyunsatisfactory.

We have now discovered a novel process for the production of Pentadienewhich eliminates the unsatisfactory practices of the art to date. Ourinvention not only provides for the economical production of pentenesfrom normal pentane by catalytic dehydrogenation but also includes thesegregation from said pentenes of pentene-l as the most suitable chargeto a second dehydrogenation treatment to produce pentadiene. By thepractice of our invention we have further discovered thatv thedehydrogenation of normal pentane over suitable catalysts may beoperated to yield larger amounts of pentene-l when pentene-2 is recycledto the catalyst along with unconverted pentane. Thus, it is an object ofthis invention to convert normal pentane to pentadiene in two stages ofdehydrogenation using a stream of substantially pure pentene-l separatedfrom the products of the first dehydrogenation step as a, charge to thesecond dehydrogenation step.

In a mixture of C5 hydrocarbons such as is produced by dehydrogenatingnormal pentane and comprising pentene-l, pentene-2 and normal pentane,the boiling points at 760 mm. are 862 F. for pentene-l, 97.5 F. forpentene-2, and 97.1 F. for n-pentane. Up to the present, most attentionhasbeen centered on separating unconverted n-pentane from the pentenemixture, and even the very efllcient commercial fractionating equipmentavailable is not satisfactory for separating n-pentane from pentene-2 insuch a mixture. For this reason the above-mentioned solvent extractionand chemical separation processes for the segregation of pentenes havebeen attempted in spite of obvious disadvantages. However, we havediscovered that a highly satisfactory fractional distillation may beconducted to separate pentene-l from the C5 hydrocarbon mixture producedby the dehydrogenation of n-pentane. By our process the C5 hydrocarbonsremaining after the separation of pentene-l and comprising pentene-2 andunconverted n-pentane are returned to the. first dehydrogenation stepfor further conversion to pentene-l.

We have further discovered that the efliciency of our process based onthe segregation of pentene-l is not impaired by the continuous recyclingof pentene-2, but that the concentration of pentene-2 in the effluentsfrom the first dehydrogenation step does not at any time exceed theequilibrium value at any given temperature of dehydrogenation. This hasbeen found to be due to the isomerizaticn of pentene-2 during thecatalytic treatment to produce substantially the ratio of the isomericpentenes produced by dehydrogenation of pure n-pentane regardless of theamount of pentene-2 in the charge. This isomerization accompanyingdehydrogenation enhances the value of our process by promoting theultimate conversion of substantially all the C5 hydrocarbons in our rawfeed stock, and by assuring a feed of substantially constant pentene-lcontent for our fractional distillation step.

In its broader aspects, our invention comprises conducting theconversion of n-pentane to pentadiene by a series of steps listed belowin the order employed: (1) catalytic dehydrogenation of n-pentane; (2)separation of the Cs hydrocarbon mixture so produced by means offractional distillation into an overhead fraction comprisingsubstantially pure pentene-l and a bottoms fraction comprising pentene-2and n-pentane; (3) continuously recycling the bottoms fraction to theinitial dehydrogenating unit, together with fresh n-pentane, andproducing thereby additional pentene-1, both by dehydrogenation ofn-pentane and by isomerization of pentene-2; (4) subjecting the overheadtraction comprising pentene-l to a second catalytic treatment undersuitable conditions to eifect a considerable dehydrogenation topentadiene; (5) separating the pentadiene so produced by any convenientmeans; and (6) recycling the unconverted pentenes remaining afterseparation of the pentadiene to either or the dehydrogenation steps asmay be convenient.

If we use the Cs fraction from refinery cracked gases instead ofn-pentane in our process, the steps of our invention are not materiallyaltered. In such a fraction comprising n-pentane and pentenes, thepentene concentration usually is not great enough to justify preliminaryfractionation for the segregation of pentene-l, and the entire stock isthen dehydrogenated by the first step of our process to produceadditional pentenes prior to the separation of the pentene-1 fraction.Obviously, it such a Cs fraction is rich enough in pentenes toapproximate or exceed the pentene content resulting from the initialdehydrogenation step we may first separate pentene-l by fractionaldistillation, and then return the pentene-2 and n-pentane as charge tothe initial dehydrogenation operation.

In order that the invention may be more clearly understood, referencewill be made to the drawing, which is a flow diagram according to whichthe steps of the invention may be carried out.

In the drawing, the raw n-pentane or suitable Cs hydrocarbon feedcomprising n-pentane and pentenes enters by line 2| into heater I wherethe feed stream is raised to the desired temperature. The hot vaporsthen pass by line 22 into catalyst cases 2. These cases contain acatalyst capable of eflecting the desired degree of dehydrogenation ofn-pentane to yield pentenes. From 2, the treated vapors pass with somecooling (not shown) through line 23 into polymer separator I where smallamounts of heavy material are removed by line 24. From I the vapors passthrough line 26 with required compression and/or cooling (not shown)into Iractionating column 4. In 4 a fractionation is effected to removehydrogen and C4 and lighter hydrocarbons overhead while the C5hydrocarbons constitute the bottoms fraction. The overhead fractionleaving by line 26 may be sent to further processing units through valve21, or a portion may be returned by line 28 into the raw pentane streamahead of the heater, providing the quantity of hydrogen gas thusreturned is not allowed to pyramid in a fashion unfavorable to thedehydrogenation reaction. The Cs fraction leaves column 4 by line 29 andis passed to iractionating column 5 wherein a fractional distillation iscarried out to take pentene-1 overhead, while pentene-2 and n-pentaneare removed from the kettle by line 3| and recycled to the raw feedstream ahead of the heater. The pentene-l traction passes through lineGI and is collected in storage 8. The auxiliary equipment for columns 4and 5, including heat exchangers, condensers, reflux accumulators andthe like is familiar to the art, and thus is not shown in this flowdiagram.

From storage I, the pentene-l concentrate passes by line 32 into aheater 1, where the stream is heated to the temperature required for thesecond dehydrogenation. The heated vapors pass I by line It to catalystcases I containing a suitable dehydrogenation catalyst. The treatedvapors exit through line 24 with some cooling, and into polymerseparator I, wherein small amounts of heavy material are removed throughline 35. From 8, the stream passes through line 20 into fractionatingcolumn ll after suitable compression and cooling (not shown). In columnll,-

hydrogen and hydrocarbons including butane and lighter are removedoverhead through line 21 while Cs hydrocarbons constitute the kettleproduct. The overhead product Irom I! may be passed to furtherprocessing units through valve II, or optionally a portion or acomponent thereof may be sent through line II to the feed stream aheadof heater I to serve as a diluent. In the latter operation, the quantityof hydrogen gas recycled is regulated so as not to influence thereaction uniavorably. The Cs fraction from column ll passes through line40 to the pentadiene extractor where pentadiene is removed by suitablereagents. The unconverted mono-olefin leaves the extractor through line4| and is recycled to the second dehydrogenation step into line 32 aheadof the heater I. The pentadiene in combination with the extractingmedium is taken through line 42 to a suitable desorbing or recovery unit(not shown).

In the operation of the first dehydrogenation step, the hydrocarbonvapors may be subjected to two or more successive treatments withdehydrogenation catalyst in a series of catalyst chambers, or the vaporsor any fraction thereof may be recycled with the fresh teed vaporsthrough the catalyst chamber. This may be accomplished, if desired, bysplitting the stream of hot treated vapors leaving the catalyst towerwith one part passing through a compresosr or its equivalent wherein thepressure is raised enough to force the recycled vapors into the streamof heated vapors prior to passage into the catalyst tower. Someadditional heat, also, may be supplied to the recycled vapors, ifdesired.

Other possible arrangements of the conventional equipment used in thepractice of our invention will be apparent to those skilled in the art,and thus are held within the scope of our invention. Also, theconditions of temperature, pressure, flow rate and the like used inoperating this equipment will depend largely on the selection of thecatalyst to be used and on the desired degree 01 conversion, since eachcatalyst has a specific range of conditions within which it operateswith maximum efliciency.

Many catalysts have been found for the dehydrogenation of hydrocarbons,and some of these may be used more or less successfully. Among the typessuggested are metals, metallic oxides, particularly diflicultlyreducible oxides, but including oxides of metals in groups 11 to VIIIinclusive of the periodic table, actiyated or lustrous carbon, clays,some silicates, and many others. The great variety of oxide catalystsmakes them of the most importance.

In the practice of our invention the charging a,ss1,ses q 3 stock to theinitial dehydrogenation operation is normally heated to temperatures inthe range 850 to 1150 F. and passed over the catalyst at such velocitiesthat contact time is quite short,

of the order of 0.5 to 10 seconds. Pressures only slightly aboveatmospheric, from about to 50 pounds are normally used, although higherpressures, up to 200 or 300 pounds gage may be used, if desired.Conditions of operation are selected with reference to economic andtechnical factors in any given installation.

In the second dehydrogenation step of our process, the charge stock isheated suiliciently to maintain temperatures between about 1050 and 1250F. in the catalyst cases. The catalysts used may be those which give asuitable degree of conversion of pentene-l to pentadiene and do notinduce excessive polymerization or cracking reactions. Further, it isusually desirable to maintain low partial pressure of pentene-l in thecharge to the second dehydrogenation step, for example, by addition ofan inert diluent in order to suppress deleterious side reactionsinvolving pentene-l.

Ordinarily two or more catalyst cases would be provided. Those cases noton stream will generally be under preparation for subsequent use, eitherby replacement of spent catalyst or by regeneration. Regeneration iscontemplated whenever the activity has declined to any predeterminedlevel, The regeneration may be car ried out by means such as controlledtreatment with an oxygen-containing gas.

The following example will serve to further illustrate one method ofpracticing our invention.

Example Normal pentane was charged to the system diagrammed in thedrawing, operated at a pressure of 30 pounds gage. The heated vaporspassed through the catalyst cases which were maintained at a temperatureof 1050 F. at the inlet and 1020 F. at the outlet. The catalyst used wasbauxite-chromium oxide. A flow rate of 1.5 liquid volumes of pentane"per hour per volume of catalyst was used and the total unsaturated Cthydrocarbons in the dehydrog'enated vapors after the recycle volume hadbeen built up averaged about 30 per cent or the charge and pentene-laveraged about per cent.

The eiiluent vapors were cooled and condensed and fixed gases wereseparated. The condensate was fractionated in two columns to removebutane and lighter overhead in the first while pentene-l I was takenoverhead in the second. The bottoms fraction 01' pentane and pentene-2was recycled to the first catalytic step while the pentane-1 richfraction was charged to the second dehydrogenation step.

The pentene-l was diluted with 3 parts by volume of a substantiallyinert Ca-Ca hydrocarbon mixture and heated to 1080 F, for passage overthe second catalyst. This catalyst was calcined bauxite and a flow rateof 1 liquid volume of feed per hour per volume of catalyst wasmaintained with an inlet temperature of 1080" F. and an outlettemperature of 1050 F. The pressure was 5 pounds gage, Conversion of thepentane-1 amounted to about 50 per cent, and 40 per cent of thepentene-l charge was recovered as 1,3-pentadiene.

The etlluents from the second step were cooled and condensed andfractionated after the separation of light gases to separate a C2-C3fraction for recycle as diluent, and a. C4-C5 fraction from whichdiolefins were extracted. The diolefin extraction was accomplished by acuprous salt reagent, after which the diolefins were recovered and thenon-diolefinic material was recycled to the second catalyst. The C4material in this recycle was included as part of the diluent gas whensteady state conditions had been obtained by the recycling operations.

The diolefinic material recovered included some butadiene, and this waseasily separated from the pentadiene by fractionation. v

The foregoing specification and example have disclosed and illustratedthe invention, but since it is of generally wide application and thenumber of examples off results obtainable by its use might be multipliedgreatly, the scope of the invention is limited only by the followingclaims.

We claim:

1. A process for preparing pentadiene from n-pentane which comprisescontacting said n-pentane with a dehydrogenation catalyst in a firstdehydrogenation step to convert at least a portion of the pentane toolefins comprising pentene-l and pentene-2, separating pentene-2 andunconverted pentane from the eiil'uent of the first dehydrogenationstep, recycling pentene-2 and pentane to the first dehydrogenation stepfor isomerization of the pentane-2 to pentene-l and conversion ofpentane to olefins, separatin pentene-1 from the eiliuent of the firstdehydrogenation step, and contacting said pen-, tene-I with adehydrogenationcatalyst in "a second dehydrogenation step for,conversion of pentane-1 to pentadiene.

2. A process for preparing pentadiene from npentane which comprisescontacting said npentane with a dehydrogenation catalyst ,in a firstdehydrogenation step to convert at least a portion of the pentane toolefins comprising pentene-l and pentene-2, separating pentene-2 andunconverted pentane from the eflluent of the first dehydrogenation step,recycling pentene-2 and pentane to the first dehydrogenation step forisomerization of the pentane-2 to pentene-l and conversion of pentane toolefins, separating pentene-l from the eiiluent of the firstdehydrogenationstep, contacting said pentene-l with a dehydrogenationcatalyst in a second dehydrogenation step for conversion of pentene-l topentadiene, separating the pentadiene from the efliuent of the seconddehydrogenation step, and recycling unconverted pentenes to the-seconddehydrogenation step.

3. A process for preparing pentadiene from Ii-pentan which comprisescontacting said n,- pentane with a dehydrogenation catalyst in a firstdehydrogenation step to convert at least a portion of the pentan'e toolefins comprising pentene-l and pentene-2, separating pentene-g andunconverted pentane from the efliuent of the first dehydrogenation step,recycling penteneand pentane to thefirst dehydrogenation step forisomerization of the pentene-2 to pentane-l4 and conversion of pentaneto olefins, separating pentene-l from the eiiiuent of the firstdehydrogenation step, admixing a. relatively inert,

diluent with the pentene-l to obtain low partial pressur of pentene-l inthe resulting mixture, and passing the mixture into contact with adehydrogenation catalyst in a second dehydrogenation step for conversionof pentane-l to penta diene. g

4. A process for preparing pentadiene from n-pentane which comprisescontacting said npentane with a dehydrogenation catalyst in a firstdehydrogenation step to convert at least a portion of the pentane toolefins comprising pentene-l and pentene-2, separating pentene-2 andunconverted pentane from the efliuent of the first dehydrogenation step,recycling pentene-2 and Dentane to the first dehydrogenation step forisomerization of the pentene-2 to pentene-l and conversion of pentane toolefins, separating pentene-l from the efliuent of the firstdehydrogenation step, admixing with the pentene-l at least an equalvolume of relatively inert diluent comprising hydrocarbons of two tofour carbon atoms to obtain low partial pressure of pentene-l in theresulting mixture, passing the mixture into contact with adehydrogenation catalyst in a second dehydrogenation step to convert atleast a portion of the pentene-l to pentadiene, separating thepentadiene from the effluent of the second dehydrogenation step,recycling unconverted pentenes to the second dehydrogenation step inadmixture with the pentene-l from the first dehydrogenation step, andrecycling hydrocarbons of two to four carbon atoms to the seconddehydrogenation step as said diluent.

5. A process for preparing pentadiene from n-pentane which comprisescontacting a hydrocarbon mixture containing n-pentane with adehydrogenation catalyst at temperatures within the range of about 850F. to about 1150 F. in a first dehydrogenation step to convert at leasta portion of the n-pentane to oleflns comprising pentene-l andpentene-2, separating pentene-Z and unconverted pentan from the eifiuentof the first dehydrogenation step, recycling pentene-2 and pentane tothe first dehydrogenation step in admixture with fresh n-pentane forisomerization of the pentene-2 to pentene-l and conversion of thepentane to olefins, separating pentene-l from the efiluent of the firstdehydrogenation step, contacting the pentene-l with a dehydrogenationcatalyst at temperatures within the range of about 1050 F. to about 1250F. in a second dehydrogentation step to convert pentene-l to pentadiene,separating the pentadiene from the eifiuent of'the seconddehydrogenation step, and recycling unconverted pentenes to the seconddehydrogenation step in admixture with pentene-l from the firstdehydrogenation step.

6. A process as defined in claim 1 wherein the catalyst in the firstdehydrogenation step is bauxite-chromium oxide, and the catalyst in thesecond dehydrogenation step is bauxite.

7. A process as defined in claim 3 wherein the catalyst in the firstdehydrogenation step consists of bauxite and chromium oxide, and thecatalyst in the second dehydrogenation step consists of bauxite.

8. A process as defined in claim 5 wherein the catalyst in the firstdehydrogenation step consists of bauxite and chromium oxide, and thecatalyst in the second dehydrogenation step consists of bauxite.

WALTER A. SCHULZE. JOHN C. HIILYER.

